Fail fixed fuel metering device and method for providing bumpless transfer to backup

ABSTRACT

A fuel metering device ( 101 ) and method for metering fuel to an engine ( 129 ) provides bumpless transfer to a backup mode. A common cavity ( 125 ) provides pressure relief, fuel metering and bypass of fuel flow. A metering valve ( 120 ) for scheduling fuel flow to the engine ( 129 ) and the metering valve ( 120 ) is controlled in either a primary mode or a backup mode. A pressure relief valve ( 123 ) limits the maximum pump discharge pressure of a fuel pump. A bypass valve ( 122 ) maintains a constant pressure across the metering valve ( 120 ) by redirecting non-metered fuel flow back to a pump stage inlet. An electrical clutch ( 131 ) determines whether the metering valve ( 120 ) function is accomplished by said primary mode or said backup mode. Bumpless transfer to a backup is accomplished automatically.

RELATED APPLICATION

This application is a division of and claims priority to applicationSer. No. 10/171,951, filed Jun. 14, 2002 now U.S. Pat. No. 6,715,278,the entire contents of which are hereby incorporated by reference. Thisapplication claims priority under 35 U.S.C. §119(e) to ProvisionalApplication of U.S. Provisional Application No. 60/326,614 filed on Oct.2, 2001, the entire contents of which are herein incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to an electronically controlled fuelmetering system.

BACKGROUND OF THE INVENTION

Many types of aircraft, such as helicopters, typically use anelectronically controlled fuel metering system that fails at a fixedmetered level of fuel in the event of losing primary power.Additionally, upon losing primary power, it is highly desirable that thesubsequent transfer to backup mode be completed in a manner that willnot require an additional workload to be imposed on the pilot of theaircraft. A backup mode is an alternative operational mode that isexecuted when a primary (or a normal operational mode) encounters apartial or total failure. This type of smooth transfer to a backup modeis facilitated by a fuel metering system that does not change or impedefuel flow. This type of unchanging, non-impeded fuel flow is known as abumpless transfer. A bumpless transfer is also a transient free orsmooth transfer that is transparent to a user or system undergoing thebumpless transfer.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a fuel metering devicefor metering fuel to an engine provides bumpless transfer to backup.This fuel metering device comprises a common cavity for providing fuelflow pressure relief, fuel metering and bypass of fuel flow to a flow offuel entering the common cavity. A metering valve schedules fuel flow tothe engine and the metering valve is controlled in either a primary modeor a backup mode. A pressure relief valve controls and limits themaximum pump discharge pressure of a fuel pump. A bypass valve maintainsa constant pressure across the metering valve by redirecting non-meteredfuel flow back to a pump stage inlet. An electrical clutch selectivelydetermines whether the metering valve function is accomplished by theprimary mode or the backup mode; and the bumpless transfer to the backupmode is accomplished automatically.

According to another aspect of the present invention, a fuel meteringmethod for metering fuel to an engine provides bumpless transfer tobackup. The method includes forwarding fuel flow into a common cavityand providing pressure relief, fuel metering and bypassing of fuel flowin a common cavity. Fuel flow is scheduled to the engine and themetering valve is controlled in either a primary mode or a backup mode.The maximum pump discharge pressure of a fuel pump is controlled andlimited. A constant pressure is maintained across the metering valve byredirecting non-metered fuel flow back to a pump stage inlet. Anelectrical clutch is used to selectively determine whether the meteringvalve function is accomplished by the primary mode or the backup mode;and the bumpless transfer to a backup is accomplished automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawing, wherein:

FIG. 1 is a schematic illustration of one exemplary embodiment of a mainfuel control (MFC) device for achieving fail fixed fuel metering; and

FIG. 2 is a block diagram of a method for achieving fail fixed fuelmetering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the invention are disclosed in the accompanying description.Alternate embodiments may be devised without departing from the spiritor the scope of the invention.

FIG. 1 illustrates one exemplary embodiment of the present invention asapplied to a rotary wing aircraft, such as a helicopter. In theembodiment of FIG. 1, the MFC device 101 performs various fuelmanagement and control functions when operating in conjunction with asingle channel full authority digital electronic control (FADEC) 100.

There are at least two electrical connector receptacles provided on theMFC device 101 to provide electrical power and signal connections to theMFC device 101 from external devices connected to it, including at leastan airframe connector 104 and a FADEC connector 102. The airframeconnector 104 carries electrical power and signals for a manual beepermotor 111 (disclosed below) and an electrical clutch 131. The FADECconnector 102 carries driver current and feedback signals for a primarymode stepper motor 109 and driver current for a solenoid 103.

The electrical devices and components of the MFC 101 can be grouped andoperated in an air environment under a common electromagneticinterference (EMI) cover (not shown). This packaging configurationfacilitates any potential upgrade to a dual channel system. For certainapplications, a dual channel system is used, whereby dual (two) primarystepper motors and associated feedback devices are operated in parallelin order to achieve higher reliability levels and more fault tolerantsystems. If one primary stepper motor or position feedback deviceexperiences a failure, a second primary stepper motor or positionfeedback device takes over, without reverting back to the manual beepermode.

The MFC device 101 is composed of a primary mode fuel metering sectionand a backup mode fuel metering section. These sections of the MFCdevice 101 are those portions of the system that perform the differentmodes. The primary mode is defined as the operation mode of the MFCdevice 101 working under FADEC 100 control. The primary mode is anautomatic mode of operation that does not require manual intervention.The components making up the primary mode fuel metering section includethe electrical connector 102, solenoid 103, pressuring valve 105, motorposition feedback control 106, damping restrictor 107, cam 108, steppermotor 109, maximum flow adjustment of the metering valve 117, fuel pumpgear stage 118, minimum flow adjustment of the metering valve 119,metering valve 120, screen 121, bypass valve 122, pump pressure reliefvalve 123, common cavity 125, metered flow discharge 128, torque tubelinkage arrangement 133, ambient pressure vent 135, and dampingrestrictor 137.

The backup mode is defined as the operation mode of the MFC device 101working under the control of a pilot in a cockpit 113. The backup modeis a manual mode of operation that requires pilot intervention through aswitch or switches in the cockpit 113. The components making up thebackup mode fuel metering section include the electrical connector 102,solenoid 103, electrical connector 104, pressuring valve 105, dampingrestrictor 107, cam 108, beeper motor 111, maximum flow adjustment ofthe metering valve 117, fuel pump gear stage 118, minimum flowadjustment of the metering valve 119; metering valve 120, screen 121,bypass valve 122, pump pressure relief valve 123, common cavity 125,metered flow discharge 128, electrical clutch 131, torque tube linkagearrangement 133, ambient pressure vent 135, and damping restrictor 137.

The MFC 101 delivers a metered fuel flow discharge 128 to an engine 129in response to FADEC 100 electrical signals in the primary mode and inresponse to a pilot activated electrical beep switch 115 in the backupmode. The beep switch 115 is manually operated by the pilot and controlsthe manual beeper motor 111 in the backup mode. Beep motors are wellknown to those skilled in the helicopter art and allow the pilot tomanually beep up or beep down the speed of the rotor.

Fuel from a fuel tank or fuel supply 130 is discharged via a fuel pumpgear stage 118 and enters the MFC device 101 where it is filteredthrough a screen 121 before entering a common cavity 125. This filteringscreen 121 can be a high pressure fuel inlet screen and captures anycontaminants or particles that may be present in the fuel. The commoncavity 125 supplies pressure relief to the fuel flow through a pumppressure relief valve 123, metering of the fuel flow by a metering valve120, and permits a proportional flow of fuel through a bypass valve 122,if there is an excess of fuel causing an undesired pressurization levelof the fuel in the MFC.

The pump pressure relief valve 123 limits the maximum pump dischargepressure exiting from the common cavity 125. Any fuel flow that is notneeded during operation exits the MFC device 101 to the pump interstage127 and recirculated through the fuel pump. A fuel pump typicallycontains two stages: a boost stage and a gear stage. The boost stagereceives fuel from a fuel tank and discharges it to the gear stage. Thetype of boost stage used can vary depending upon the performancerequirements of the pump, but it essentially conditions the fuel goinginto the gear stage by increasing or boosting the pressure level.Bypassed fuel from the MFC device 101 enters the pump interstage 117between the boost and gear stages.

In FIG. 1, ambient pressure equalization of the compartment housing thesolenoid 103, the beeper motor 111, the stepper motor 109 and a steppermotor position feedback 106 (discussed later) is maintained by theambient pressure vent 135.

The proportional bypass valve 122 maintains a constant pressure (P)across the metering valve 120 by sensing pressure P1 141 and pressure P2143 that exist across the metering valve 120 by redirecting non-meteredflow back to the pump interstage 127. Pressure P1 141 is measured at oneend of the proportional bypass valve 122 and P2 143 is measured at theopposite (spring) end of the valve. The metering valve 120 schedulesflow to the engine as commanded by either the primary mode or backupmodes of operation. The fuel flow within the MFC device 101 iscalibrated and sized to a specific engine size and configuration byusing a maximum flow adjustment mechanism 117 and a minimum flowadjustment mechanism 119. An electrical clutch 131 selectivelydetermines whether the metering valve 120 function is accomplished bythe primary or the backup system. Those skilled in the art willappreciate that a clutch, such as the electrical clutch 131 shown inFIG. 1, applies a clamping force that engages or disengages disks whichare attached between a motor drive shaft and a clutch output shaft.

In the primary mode, fuel metering is controlled by an electrical signalsent from the FADEC 100 to the stepper motor 109. The stepper motor 109rotates a two-dimension cam 108 that performs a dual function. Onefunction of the cam 108 is to provide a contour that positions themetering via a torque tube that forms part of a metering valve linkagearrangement 133. Another function of the cam 108 is to provide aposition feedback contour for an electrical position sensor to maintainclosed loop motor position feedback control 106 of the stepper motor109. A metering valve 120 linkage arrangement 133 is mechanicallypreloaded against the cam 108 contour. The holding torque of the steppermotor 109 retains the metering valve 120 in a fixed position when thestepper motor 109 is not powered.

A transfer into the backup mode is accomplished when the power isapplied to the electrical clutch 131 connected to the beeper motor 111.The beeper motor 111 is connected to a manual control mechanism 115 thatthe pilot uses to beep up (increase metered fuel flow) or beep down(decrease metered fuel flow) the beeper motor 111.

The transfer into backup mode can be accomplished in one of two ways.The transfer may occur automatically as scheduled by the FADEC 100 inthe event of the loss of primary power or manually at the pilot's optionby a switch in the cockpit 113. This action engages the backup DC motor111 with the cam 108. The beeper motor 111 connects to the electricalconnector 104, which then provides an electrical connection to thecockpit 113.

An electrical switch 113.1 in the cockpit 113 sends electrical signalsto a DC motor 111 to rotate the cam 108, back drive the stepper motor109, and thereby control the metered flow of fuel. The DC motor 111speed and reduction gearing are selected to limit the rate of fuel flowchange and the metered flow of fuel discharged 128 to the engine 129.This helps prevent surge or flameout of the engine 129 connected to theMFC device 101.

Fuel flow discharged from the metering valve 120 passes through thenormally open solenoid 103. This solenoid 103 controls a valve thatterminates and bypasses metered flow to the pump interstage 117 whencommanded by an electrical signal from the electronic control.

Typically, the manufacturers of engines want to have the capability toterminate fuel flow to the engine for either one of two reasons. Onereason is that the pilot has landed and wants to shut off the fuel flowto an engine. Another reason is that an occurrence of an engineoverspeed condition has been sensed. If the pilot wants to shutoff fuelflow to the engine, the pilot uses a switch to send an electrical signalto the FADEC 100 which in turn energizes the solenoid 103 to terminatefuel flow. If the FADEC 100 senses an engine overspeed condition, theFADEC 100 in communication with the solenoid 103 automaticallyterminates the fuel flow. Damping restrictors 107 and 137 are selectedand sized to provide stability and damping of the fuel flow passingthrough the restrictors.

This feature of returning metered flow back to pump interstage duringshutoff prevents dead heading of a fuel pump and subsequent forcedoperation on the pressure relief valve 123 during shutdown. Dead headingof a fuel pump results from the following situation. If the solenoid 103suddenly terminates fuel flow, there is a brief increment of time whenthe fuel pump is pushing more fuel flow into the 118 region shown inFIG. 1 than the bypass valve 122 is able to handle. The solenoid 103(includes a valve as part of the solenoid 103 as shown in FIG. 1)controlling a valve has an operation or reaction speed that operatesmuch faster (e.g., 5 times faster) than the bypass valve 122. Thisproduces a momentary dead heading of the pump and the pressure riseshigh enough to activate the pressure relief valve 123. In other words,the pump would be dead headed if all of its discharge flow had nowhereto go.

Fuel flow exits the MFC device 101 after it passes through a pressuringvalve 105. The pressuring valve 105 has two functions. The pressuringvalve 105 provides a drip tight seal until the solenoid 103 controlledvalve has been opened and the fuel pump 118 has generated apredetermined pressure. The pressurizing valve 105 also allows thebypass valve 122 to stay in regulation at low nozzle manifold pressureswhen pump inlet pressures are high.

In FIG. 2 a method for the fuel metering device is described. In step200, fuel flows into the system from an external source or reservoir. Instep 201, a common cavity provides fuel flow pressure relief, fuel flowmetering and fuel flow bypass. In step 202, the fuel flow is scheduleddepending upon whether the system is in a primary or backup mode. Instep 203, maximum pump discharge pressure is controlled using a pressurerelief valve. In step 204, constant pressure is maintained in the systemby maintaining a constant pressure across a metering valve byredirecting non-metered fuel flow back to a pump stage inlet. In step205, an electrical clutch selectively determines whether the meteringvalve function is accomplished by the primary or backup mode. In step206, the bumpless transfer to backup mode is accomplished automatically.

1. A fuel metering method for metering fuel to an engine providingbumpless transfer to backup comprising the steps of: forwarding fuelflow into a common cavity; providing pressure relief, fuel metering andbypassing of fuel flow in said common cavity; scheduling fuel flow tosaid engine in accordance with either a primary mode or a backup mode;controlling and limiting the maximum pump discharge pressure of a fuelpump; maintaining a constant pressure across a metering valve byredirecting non-metered fuel flow back to a pump stage inlet; and usingan electrical clutch to selectively determine whether the metering valvefunction is accomplished by said primary mode or said backup modewherein said bumpless transfer to said backup mode is accomplishedautomatically.
 2. The fuel metering method of claim 1 wherein fuelmetering in said primary mode is controlled by an electrical signal froma full authority digital electronic control (FADEC) system.
 3. The fuelmetering method of claim 1 wherein transfer into said backup mode isautomatic upon loss of primary power and is selectable by an electricalswitch in a cockpit.
 4. The fuel metering method of claim 2 wherein saidbackup mode operates independently from the FADEC system.
 5. The fuelmetering method of claim 2 wherein fuel metering in the primary mode iscontrolled by an electrical signal from the FADEC to a stepper motor. 6.The fuel metering method of claim 5 wherein said motor rotates atwo-dimension cam.
 7. The fuel metering method of claim 6 wherein saidcam provides a contour that positions said metering valve using ametering valve linkage arrangement.
 8. The fuel metering method of claim6 wherein said cam provides a feedback contour for an electricalposition sensor to maintain closed loop control of said stepper motor.9. The fuel metering method of claim 7 wherein said metering valvelinkage arrangement is mechanically preloaded against a contour of saidcam.
 10. The fuel metering method of claim 5 wherein said stepper motorhas a holding torque and said holding torque retains said metering valvein a fixed position when said stepper motor is not powered.
 11. The fuelmetering method of claim 1 wherein transfer to said backup mode isaccomplished when power is applied to a DC motor electrical clutch. 12.The fuel metering method of claim 11 wherein said DC motor is a beepermotor.
 13. The fuel metering method of claim 12 wherein a transfer intosaid backup mode is accomplished when power is applied to an electricalclutch of said beeper motor.
 14. The fuel metering method of claim 13wherein said transfer into backup mode occurs automatically as scheduledand executed by a full authority digital electronic control (FADEC)system or manually by an electrical switch in said cockpit.
 15. The fuelmetering method of claim 13 wherein said transfer initiates an actionthat engages a backup DC motor with the scheduling cam.
 16. The fuelmetering method of claim 15 wherein an electrical cockpit switch sendselectrical signals to said backup DC motor to rotate the cam andbackdrive said stepper motor.
 17. The fuel metering method of claim 16wherein said DC motor speed and reduction gearing are selected to limitthe rate of fuel flow change.
 18. The fuel metering method of claim 17wherein said fuel flow change is controlled to prevent engine surge orengine flameout.
 19. The fuel metering method of claim 1 wherein thefuel flow discharged from said metering valve passes through a normallyopen overspeed/shutoff solenoid valve.
 20. The fuel metering method ofclaim 19 wherein said solenoid valve terminates and bypasses meteredfuel flow to a pump interstage upon receiving an electrical signal froma full authority digital electronic control (FADEC) system.
 21. The fuelmetering method of claim 1 wherein fuel flow exits said fuel meteringdevice after passing through a pressuring valve.
 22. The fuel meteringmethod of claim 21 wherein fuel flow discharged from said metering valvepasses through a normally open overspeed/shutoff solenoid valve and saidpressuring valve provides a drip tight seal until said solenoid valve ina shutoff mode has been opened and said fuel pump has generated apredetermined pressure.
 23. The fuel metering method of claim 21 whereinsaid pressuring valve allows a bypass valve to stay in regulation at lowmanifold pressure when pump inlet pressures are high.
 24. The fuelmetering method of claim 1 wherein electrical connectors connect to amain fuel control (MFC).
 25. The fuel metering method of claim 24wherein at least one electrical connector is an airframe connector andsaid airframe connector carries signals for a manual beeper motor andsaid electrical clutch.
 26. The fuel metering method of claim 24 whereinat least one electrical connector is a FADEC connector and said FADECconnector carries signals for a primary mode stepper motor and thedriver current for shutoff and manual transfer solenoids.
 27. The fuelmetering method of claim 1 wherein all electrical components areshielded under an electromagnetic interference (EMI) cover.
 28. The fuelmetering method of claim 27 wherein said fuel metering method is carriedout using a fuel metering device that is upgradeable to a dual channelsystem.